I've been to Iceland twice, and it would be very easy to do on your own. There are several companies that do multi-day tours, some more budget/luxury than others. Goecco, Extreme Iceland are two that come to mind. Driving would also be easy in the summer, as there are few roads, particularly once you get out of Reykjavik, so hard to get too lost. You can also fly from Rey to Akyreyri, the second largest city, in the north, if you wanted to base some tours from Rey and some from another location.